In a build-to-order (BTO) management system, domestic production orders are incorporated into a production plan before overseas production orders. When incorporating the production orders into the production plan, an ideal number of products is set for various equalizing factors, and several kinds of the production plans are made on the basis of an achievement ratio of the number of products for the equalizing factors actually incorporated into the production plan to the ideal production number of products and the dates of receiving the production orders. The resulting production plans are compared to choose an optimal production plan.